Heretofore, phosphites have been utilized as antioxidants for various polymers, both alone and in blends with other stabilizers. There are two basic types of antioxidants, primary and secondary. Primary antioxidants intercept and stabilize free radicals by donating active hydrogen atoms, interrupting the free-radical chain oxidation. Hindered phenols and aromatic amines represent the two main types of primary antioxidants. Secondary antioxidants prevent formation of additional free radicals by decomposing the unstable hydroperoxides into a stable product, thus removing a source of free radical formation which accelerates the oxidation. Phosphites and thioesters are secondary antioxidants that function by decomposing hydroperoxides, thus preventing free-radical formation. Secondary antioxidants are often used along with primary antioxidants, but can be used alone, especially if they contain a hindered phenolic group within their structure. Together they decrease the discoloration of the polymer and may also regenerate the primary antioxidant.
In U.S. Pat. No. 4,187,212, Zinke et al. disclose that a mixture of aryl-containing phosphites and ortho-substituted phenols in polyolefins exhibits a particularly good stabilizing effect. This particular degree of effectiveness is reflected especially in the excellent absence of discoloration in the resulting polymers. In this case, the phosphites exemplified are tris-aryl phosphites, which are known from the prior art to have good hydrolytic stability. The stabilizer mixtures, in this case, are physical mixtures of powders and are incorporated into the polymer by powder blending and subsequent processing in a kneading machine, mixing rolls, or extruder.
As set forth in U.S. Pat. No. 4,957,956, a solid stabilizer composition for organic polymers is formed by a mixture of a solid, continuous phase of a phenolic antioxidant, namely tetrakis[3-(3,5-di-tert-butyl4-hydroxyphenyl)propionyloxymethyl] methane and the dispersed phase is constituted by either an amorphous or crystalline organic phosphite in the form of particles with a particle size from 10 um to 2 mm, with the weight ratio of the continuous phase to the dispersed phase within the range of 9:1 to 1:9.
The above patent pointed out that in the use of mixtures of sterically hindered phenols and organic phosphites in the stabilization of organic polymers, difficulties exist in accomplishing complete homogenization between the two stabilizers, and between these and the organic polymer. With blends of solid powdered stabilizers, intimate mixing is often not achieved, and often the stabilizing effect of the stabilizers shown does not reach the maximum possible level. Often this fact, combined with the phenomenon of hydrolysis of the phosphite is subject to, does not allow as high a stabilizing effect as desired. When certain organic phosphites are introduced to the polymer as a dispersed (solid particulate) phase in a continuous amorphous phase of tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl] methane, a few of said phosphites are endowed with exceptionally high characteristics of resistance to hydrolysis.
In the case of U.S. Pat. No. 4,957,956, the stabilizer composites are produced by mixing the powders of the organic phosphite and the tetrakis [3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl] methane, then under nitrogen or another inert gas, the mass is raised to a temperature which is typically on the order of 160° C. to 170° C. The mass is homogenized and is then submitted to a sudden cooling down to temperature values equal to room temperature (20° C.–25° C.).
U.S. Pat. No. 5,155,153 describes a stabilizer composite for organic polymers containing a three-component mixture of a sterically hindered phenol, a triaryl phosphite, and a dialkylthiodipropionate. Stabilized compositions are made by simple addition of the three components to the polymer.
U.S. Pat. No. 5,844,042 relates to a process for obtaining granular forms from mixtures of powders of two or more additives for organic polymers, which comprises extruding the additive mixture at a temperature between the melting point of the component with the lowest melting point and about 140° C., with the condition that, when the mixture consists of two additives, these are not tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl] methane, either in a crystalline or amorphous phase, and an antiacid. Other attempts to produce effective stabilizer blends for the stabilization of organic polymers are described, e.g., extruding a mixture of at least two components below the melting point of the highest melting component and above the melting point of the lowest melting component.
In U.S. Pat. No. 5,845,656, a mixture of stabilizers is made in a pelletized form from a homogenous mixture of at least one stabilizer and an agent which will prevent melting of the stabilizer. The stabilizers are antioxidants such as organic phosphites and hindered phenols, hindered amines, U. V. light stabilizers, or combinations thereof. If the mixture is permitted to melt, pellets of the stabilizer mixture are not able to be produced by the method of this patent.